A great deal of recent research and development has focused on improving organic EL displays. Self-luminous organic EL display devices are advantageous for achieving high contrast. Further, because organic EL elements have a high response rate, it is possible to display video including rapid movement without blur.
Currently, active matrix type displays, in which an organic EL element is driven by a thin film transistor (TFT), have become, in response to a demand for a high-definition and high-resolution display, the most commonly employed. Such displays are typically manufactured with an organic EL element disposed on a substrate on which a low temperature polysilicon TFT or the like is formed. Although a low temperature polysilicon TFT is suitable for use as a drive element of an organic EL display because of the TFT' s high mobility and stable operation, there is still a problem of wide variation of characteristics such as the threshold or mobility. Uneven luminance becomes noticeable in a display when a constant current drive is applied in a saturation region resulting in non-uniformity among pixels. Thus, digital drive methods for reducing display unevenness by operating a TFT as a switch in a linear region are disclosed. For example, in the digital drives disclosed in JP 2005-275315 A (Patent Document 1) and JP 2005-331891 A (Patent Document 2), a plurality of subframes are used to achieve multiple gray levels because pixels are controlled in two values, illuminating or not illuminating (subframe type digital driving).